The present invention relates to a semiconductor light emitting element which is provided with an external modulation region independent of a light emitting region.
The optical fiber transmission technology, which is now being intensively developed, is to transmit information in accordance wtih the intensity of light. On the other hand, there is a growing tendency to the so-called coherent optical communication which transmits information by modulating the frequency or phase of light, and experiments utilizing low-loss optical fiber transmission and space propagation are being conducted by some researchers. Conventionally, the frequency or phase of light is modulated by employing an optical modulation element separately of a light emitting element, i.e. a semiconductor laser, or by a direct modulation method which varies the current to be injected into the semiconductor laser, or a method which partially modulates the current to be injected into a light emitting layer in a distributed feedback semiconductor laser (hereinafter referred to simply as a DFB laser). However, the former method calls for the insertion of an optical isolator between the semiconductor laser and the optical modulation element, and suffers a very great loss which is caused by the optical coupling of those elements. The latter methods perform the modulation by the injection current so as to cause a variation in carrier density in the laser region, and hence it encounters difficulty in precisely modulating the frequency or phase of light while retaining a narrow oscillation spectral width.